An analog-to-digital converter (ADC) converts an analog signal to a digital signal. The analog input range of the ADC is divided into a 2n discrete steps, where “n” is the number of bits of the output digital signal from the ADC. For example, if n is 5 (for a 5-bit ADC), then the converter discretizes the analog input signal into 25 or 32 discrete steps.
One type of ADC is a flash converter which includes a separate comparator for each of the discretized elements of the input analog range. In the example above of a 5-bit flash converter which has 32 discrete steps, the converter would have 32 separate comparators. A separate reference voltage is generated and provided to each comparator to compare with the analog input range.
To save power and area, a two-step flash converter includes a zero crossing comparator to initially determine whether the analog input is positive or negative to thereby determine in which half of the analog input range that the analog input signal resides. Then, a flash converter is used to produce the digital output value. As the zero crossing comparator has already ruled out half of the analog input range for the particular analog input signal to be converted, the subsequent flash converter need not include 32 comparators, and instead only includes 16 comparators. The reference voltage provided to each of the comparators is configured by the flash converter to be a positive voltage or a negative voltage.